The detection of the commencement of the opening of the nozzle needle or of the point in time of the abutment of the armature on the nozzle needle is based on several principles. The eddy-current driven coupling between the mechanics (armature and injector needle) and the magnetic circuit (coil) generates a feedback signal based on the displacement of the mechanics. A speed-dependent eddy current is induced in the armature as a result of the displacement of the nozzle needle and of the armature, which also causes a reaction on the electromagnetic circuit. Depending on the speed of displacement, a voltage is induced in the electromagnet that is superimposed on the activation signal. The utilization of said effect requires the superimposition of the basic electrical variables voltage or current with the signal change owing to the armature and/or the needle displacement to be suitably separated and then to be further processed. In doing so, the characteristic signal shape in the voltage or current signal is analyzed in relation to the point in time of occurrence.
The following methods for detecting a characteristic signal profile during the opening process are known:
Current Measurement Method
This method requires active influencing of the current profile (the standard activation form) in order to ensure that the magnetic circuit is not saturated. With said measurement techniques, a needle stop measurement signal can only be detected in the event of full drive.
Voltage Measurement
This type of measurement is not possible with the standard activation form because voltage imposition overwrites all characteristics. In doing so it is assumed therefrom that the electromagnetic circuit is controlled with sample-and-hold activation with a boost phase.
If the injector is activated with the standard voltage profile, the nozzle needle is opened very rapidly. No signal is generated in this case because the nozzle needle abutment takes place at a point in time at which the magnet circuit is saturated. There is therefore no signal available for detecting the commencement of the opening of the nozzle needle.
The nozzle needle abutment can only be detected if an activation profile is used with which the nozzle needle abutment takes place when the magnetic circuit is not in saturation. This can be achieved by reducing the needle opening rate, wherein however operation with such a detection profile cannot be carried out permanently because the reduced nozzle needle speed can result in a lower injection quality (atomization, emissions etc.). With such a procedure, the quality of the injection would therefore have been affected.
These and other previously known methods for generally determining the opening or closing time of an electromagnetically driven device use either a measurement channel for the determination of injector opening and closing (current/voltage measurement), with intervention into the energization during the detection of opening, or current measurement alone for detection of the opening and closing times. The intrusive intervention into the basic activation of the coil and the limitations associated therewith, result in altered injection behavior.